1. Field of the Invention
The present invention relates to a charged-particle detecting apparatus which detects charged particles such as electrons and ions as a detector to be applied to time-of-flight mass spectrometry or the like.
2. Related Background Art
As a method for detecting a molecular weight of a polymer, time-of-flight mass spectrometry (TOF-MS) is known. FIG. 1 is a drawing for explaining this TOF-MS.
As shown in FIG. 1, in the TOF-MS, a detector 100 is set on one end in a vacuum vessel 110, and a sample 120 is arranged on the other end in the vacuum vessel 110. Between these, an electrode 130 having an opening is arranged. The electrode 130 is grounded, and when a predetermined voltage is applied to the sample 120, ions emitted from the sample 120 are accelerated by an electric field formed between the sample 120 and the electrode 130, and collide with the detector 100. The acceleration energy to be given to the ions between the sample 120 and the electrode 130 is determined according to ion charge. Therefore, when the ion charge is the same, the speed when passing the electrode 130 depends on the weight of ions. Between the electrode 130 and the detector 100, ions fly at a constant speed, so that the flight time of the ions from the electrode 130 to the detector 100 is in inverse proportion to the speed. That is, by calculating the flight time from the electrode 130 to the detector 100, the weight of ions can be judged.
As such a detector, for example, the detector disclosed in Japanese Patent Application Laid-Open No. 06-28997 (Document 1) is applicable. FIG. 2 is a schematic cross-sectional view showing an example of a detector applicable to TOP-SM. In the detector 100a shown in FIG. 2, two micro channel plates (MCP) 20 and 21 (hereinafter, referred to as a MCP group 2) are sandwiched between an IN electrode 1 and an OUT electrode 3 which have openings in their central portions. In front of the IN electrode 1, a wire-mesh grid electrode 106 retained by a frame 105 is arranged, and on the other hand, behind the OUT electrode 3, an anode electrode 4 is arranged. To the shielding side of a signal reading BNC terminal (Bayonet Neil-Concelman connector) 60, a casing 5x comprised of a conductive material is connected, and on the other hand, to the core 601 side, an electrode 47 is connected. Between the casing 5x and the OUT electrode 3, and between the electrode 47 and the anode electrode 4, dielectric bodies 22 and 46 are arranged, respectively, to form a capacitor.
In the detector 100a structured as described above, when charged particles are made incident on the MCP group 2, many electrons (secondary electrons multiplied by each MCP) are responsively emitted from the MCP group 2. The secondary electrons thus emitted arrive at the anode electrode 4 and are converted into an electric signal as a voltage or current change (signal is outputted from the core 601). At this time, a capacitor is formed between the anode electrode 4 and the core 601, so that the detection signal is outputted to the outside at a ground potential, and the capacitor formed between the casing 5x and the OUT electrode 3 suppresses waveform distortion or ringing of the output signal.